Oyster Aquaculture Oyster Biology Oysters are mollusks shelled invertebrates (or animals without backbones) in the same zoological phylum, Mollusca, as the mussel, clam, abalone, snail, octopus and squid. Oysters are also classified as bivalves. An elastic ligament hinge in two parts, or valves, holds their shells together. An oyster makes its own shell, secreting calcium and other materials from glandular tissue in its soft flesh. Worldwide, there are more than 100 living oyster species. Wholly adapted for an aquatic existence, oysters have gills for breathing. As water is drawn across the gills, these feathery organs also collect particles of food from the water that surrounds them. Some species live in deep enough water to avoid being exposed to the air at low tide. Those that settle in shallower seas must deal periodically with exposure to the elements. For protection, they clamp their two-part shells tightly together, using their powerful adductor muscles to hold tight (the dark spots on the inside of an empty shell are the points where the ends of this muscle once adhered). Now sealed, the oyster can retain enough moisture to survive for hours, or, if necessary, several days out of water. Oysters in the genus Crassostrea shed sperm or eggs into the water, where fertilization, hatching and larval development take place. Once in the water, larvae become part of the planktonic community and gradually undergo several changes. After three to four weeks, the larvae metamorphose to the juvenile form and are ready to settle on and attach to suitable substrates. At this time, oyster growers place various kinds of material into the water to catch the larvae, in essence providing the young oysters with a home. This process is called cultching. The catching materials are referred to as cultch and the recently settled oysters are known as spat. Old shells from previously harvested oysters are commonly used as cultch. [https://wsg.washington.edu/wordpress/wp-content/uploads/publications/small-scale- Oyster-Farming.pdf] Reproduction and development Oysters are dioecious, with sexes separate; however, they exhibit alternate sexuality. Oysters usually begin life as a male, change to female then possibly back to male. This phenomenon is known as protandric hermaphroditism. In the first spawning season most young individuals are males. After the second spawning season the numbers of individuals in the total population of each sex are almost equal.
Life cycle of the Eastern oyster, Crassostrea virginica [http://aqua.ucdavis.edu/databaseroot/pdf/432fs.pdf] The sexual cycle begins with spawning which is triggered by water temperature. Gametogenesis, the maturation of ova and sperm, follows the spring s increase in temperature culminating in ripeness when water temperatures trigger the spawning threshold. The American oyster spawns at temperatures greater than 16 C (60.7 F). The next stage in the reproductive cycle is the fertilization of the egg by the sperm which are released at the time of spawning by the female and male, respectively. During this stage the female produces millions of eggs. The ripe eggs of the American oyster are pear-shaped, but become globular after fertilization. Spawned eggs are heavier than water and quickly settle to the bottom where they are transported by currents and waves. The egg stage is brief, remaining immersed until development, which begins immediately after fertilization. After fertilization, cell division proceeds rapidly and within hours, the swimming trochophore stage is formed and the oysters are called veliger larvae. Within 24 hours two thin valves have been formed, and within 48 hours, the shell fully encloses the body and the elementary organ systems have been formed. Larval development generally lasts 10 to 21 days depending on culture techniques. The pelagic larvae of oysters act to distribute the species. Currents are the major influence on the dispersal of oyster larvae in the wild. When the veliger develops two eye spots that are on each shell, it is called an eyedpediveliger. The eye spots aid in selecting an acceptable location for attachment. Soon after, a ciliated, muscular foot is formed. These pediveligers become bottom oriented, crawling on substrate to find a suitable site to cement or set. At this time they are negatively phototactic (response to light), generally setting on darkened surfaces using
their cement glands. Once set, metamorphosis of internal organs occurs and the velum, eye spots and foot are lost. Once a small amount of cementing fluid is excreted from the edge or bill of the larval shell, the spat become sedentary filter-feeders through adulthood. [http://aqua.ucdavis.edu/databaseroot/pdf/432fs.pdf] Seed Sources The two primary sources of seed oysters are naturally occurring seed and hatchery seed. Collection of natural spat is accomplished by placing bags of clean oyster shell or other cultch material in the water prior to the forecasted settlement of the planktonic oyster larvae. The cultch is left in the water for several weeks. If the collection is successful, the spatted cultch is examined and transported to a nursery area. It is then handled similarly to hatchery cultch. It should be noted that the yearly setting frequency has always been unpredictable. In some years, catches exceed 50 spat per shell. In other years, none. Hatchery production includes the conditioning and spawning of adult oysters, setting the spat and growing the young oysters in nursery tanks. The majority of oyster hatcheries produce either bags of cultched oyster seed or single (cultchless) oyster seed. So-called eyed larvae, which are ready to settle onto cultch, can also be acquired from the hatcheries. [https://wsg.washington.edu/wordpress/wp-content/uploads/publications/small-scale- Oyster-Farming.pdf] Triploid Oysters One day in 1979, a young graduate student was sitting hunched over a microscope in the attic of a hatchery when he realized he had created a new kind of oyster, an oyster nature had never designed. For more than a year Allen had been testing techniques for forcing additional chromosomes into the Eastern oyster, Crassostrea virginica, the native species that grows along the Atlantic and Gulf coasts of America. While nature gave oysters two sets of chromosomes, making them diploids, Allen was trying to pack his oysters with three sets of chromosomes, making them triploids. Those extra chromosomes would help Allen's oysters grow fat faster, and those qualities, in theory, should quickly turn these triploid seed oysters into a moneymaker for oyster farmers not just in Maine but in any other coastal waters where oyster aquaculture was an option. A triploid oyster, with its triple set of chromosomes, was designed to avoid the market drawbacks of traditional oysters. Nature's oysters are diploids, and they are seldom sold or eaten during the summer months when they're growing gonads to produce sperm and eggs. Those are the months, according to custom and common sense, when oysters are seldom good eating: too crammed with gonads or too watery after spawning. Come September, the first of the "R" months, spawned-out oysters are beginning to recover and fatten up with meat, and watermen and oyster growers can finally bring them to market.
Triploids are sterile oysters. They usually don't grow gonads and don't bother spawning, letting them put all their energy year-round into growing meat. As a result the yield from an invented oyster is up to twice the yield from a natural oyster. [http://ww2.mdsg.umd.edu/cq/v09n2/main2/] Growing Methods Bottom culture is the most common method of oyster farming because of the low maintenance and simple preparation requirements. Cultched seed is placed onto beds, then harvested when the oysters reach appropriate (most likely medium) size. The best areas for this technique are protected bays or inlets with firm mud bottoms where currents and waves are not excessive. Floating culture uses the subtidal bottoms that are leased from the state, and application of this method is governed by local and state regulations. In this method, oyster grow-out trays or polyethylene cages may simply be stacked on the floor of a sink float, or the stack may be suspended from a raft or floating longline system in the water column. Of course, greater production levels per unit of surface area will result from the latter culture technique. If a float is used, one end should be tied to the anchor and the other end left free. This will allow the float always to swing against the tide, providing for better circulation through the culture devices. [https://wsg.washington.edu/wordpress/wpcontent/uploads/publications/small-scale-oyster-farming.pdf] Some advantages and disadvantages associated with bottom and off-bottom oyster culture techniques Advantages Less labor to maintain and stage seed for stocking growout areas Lower capital and maintenance expenses as no nursery or grow-out structures are required Less labor in moving shell stock to new areas because structures are not involved Generally, fewer permits are required for bottom culture than off-bottom culture Bottom Culture Usually less negative visual impact at nursery and growout sites Off-Bottom Culture Advantages Less impact from predators and pests Significantly less chance for problems resulting from siltation Use of total water column by three-dimensional culture techniques Spat survival on both sides of the mother shell Less expensive equipment required for harvest Disadvantages Higher percentage of mortality from predators and pests Higher potential for siltation Spat usually survive only on one side of the mother shell Public resistance to many predator control methods Disadvantages More public resistance to visual impact of culture structures Potential conflict with recreational boating Fouling organisms severely impact some container systems Labor intensive in initial set-up Capital cost and maintenance of nursery and grow-out structures Conte, F.S., S.C. Harbell and R.L. RaLonde. 1994. Oyster Culture - fundamentals and technology of the West Coast industry. Western Region Aquaculture Center, Seattle, WA.
Oyster Hatcheries Oyster hatcheries provide juvenile oysters for commercial production, restoration projects and research. There are many critical elements to an oyster hatchery, but none is more important than location, or more specifically, location relative to water supply. Oyster hatcheries require large volumes of clean seawater with salinities in the range of 15 to 30 parts per thousand (ppt). Salinity is not a major issue in many areas, but some estuaries have periodic episodes of freshwater inflows that can reduce salinity below 10 ppt. Low salinity water is not conducive to spawning, larval development, or early growth of young oysters. Turbidity, potential pollutants, watershed development, boat traffic, and natural algae production are other aspects of water quality to consider. A tank of oyster larvae at Taylor Shellfish hatchery in Quilcene, WA [https://adaptationstories.com/2013/09/04/cracking-thecase-of-the-vanishing-oyster-larvae/] Most facilities have a separate pump station that brings saltwater from a nearby source to the hatchery. Before it enters the hatchery, water is often pumped to large holding tanks where settling reduces turbidity. Or, the holding tanks can be bypassed and water pumped directly to the hatchery. The plumbing is designed with a water filtration and treatment system consisting of some combination of rapid sand filters, cartridge filters, activated carbon, ultraviolet (UV) sterilization or pasteurization. Treated seawater is then suitable for larval and algal production. Tanks for larval production are circular, generally 250 gallons (946 L) or larger, and have center drains and sloping or conical bottoms. Shallow rectangular tanks with drain pipes provide nursery space for juvenile oysters. Oyster larvae feed by filtering small, singlecell algae from the water. They must be supplied with the right size food at a density that makes the food easy to encounter. Tank design is important when considering flow-through. Larvae need to be retained within the tank and the volume large enough to ensure that added food has a sufficient residence time to be eaten. The exchange rate needs to be sufficient to prevent metabolic wastes and debris from accumulating, yet there may still be the need to flush out the tank periodically after cleaning the interior surfaces.
Setting larvae Larvae are ready to set when they have a well-developed eye spot and are 290 µm or more in length. Larvae that are ready to set are usually selected by sieving them through a 180-µm screen (254-µm diagonal opening). Larvae that pass through are restocked. The retained larvae are sieved again on a 210-µm screen (296-µm diagonal opening). Those that pass through are also restocked to a separate tank. The retained larvae (larger than 296 µm) are pooled and counted before being transferred to setting tanks. The large, eyed larvae can be set on a variety of materials (cultch) using several methods.. The two basic choices are A typical arrangement for flow-through larval producing single oysters (good tank (AR=algal reservoir; P=pump; for research and/or off-bottom F=flowmeter; D=drain) farming for the half shell [http://www.fao.org/docrep/007/y5720e/y5720e0a. market) and producing oyster htm] clusters (good for producing a lot of oysters for restoration projects and/or farming aimed at the shucked meat market). Sieving oyster larvae [http://www.thefishsite.com/articles/1596/oyst er-hatchery-techniques/] Single oysters can be obtained by setting larvae on microcultch, very smooth and slippery surfaces, or by chemical induction. Microcultch is usually made of finely ground oyster shell sieved to produce shell pieces 250 to 300 µm in diameter. A single larva sets on each particle. Oyster larvae also can be set on a slippery, flexible surface such as Mylar sheets. After larvae have metamorphosed to spat they can be popped off the sheet.
Cluster oysters are the result of setting larvae on large cultch, usually whole oyster shell. This results in a product similar to what occurs in nature a shell with many spat. Over time, natural attrition results in two to four adult oysters per shell. Upwelling and downwelling arrangements for feeding eyed larvae [http://www.fao.org/docrep/007/y5720e/y5720e0b.htm] Single oysters are produced by introducing eyed larvae (250 larvae per square inch or 100 per square cm of bottom surface area) into containers with fine mesh bottoms (l50- to 180-µm) and a thin layer of microcultch. The containers, called wellers, are immersed in shallow tanks containing treated seawater. Containers are configured so that water can flow up through (upwelling) or down through (downwellng) the mesh bottoms by routing water through an opening near the top of the container. Setting on whole shell or other large cultch can be done by placing the cultch in large mesh bags and the bags into tanks with treated seawater. Eyed larvae are introduced at the rate of 100 per shell, with a goal of obtaining 10 to 30 early spat per shell. Gentle aeration is applied, algae added, and the tank covered. After several days, the tanks are supplied with a continuous flow of coarsely filtered sea water. Bags of cultch should be washed occasionally and tanks drained to remove wastes. When oysters reach a size that protects them somewhat from predation, and before they grow through the bags, they can be removed from the bags and spread on suitable Marinetics uses oyster floats on the Choptank River [http://choptanksweets.com/aquaculture/] bottom substrates or growing cages.
Bottom oyster cages require only a small buoy on the surface [http://www.chesapeakebay.net/blog/post/not_your_ grandfathers_oyster_company] [http://www.aces.edu/dept/fisheries/education/documents/oyster_hatchery_techniques.pd f] Aquaculture Resource Guide, Maryland Aquaculture http://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/aquaculture/a RG%202015%20text.pdf Small-Scale Oyster Farming https://wsg.washington.edu/wordpress/wp-content/uploads/publications/small-scale- Oyster-Farming.pdf Oyster Biology http://ww2.mdsg.umd.edu/store/books/mdoysters/ The Cultivation of American Oysters http://aqua.ucdavis.edu/databaseroot/pdf/432fs.pdf Oyster Hatchery Techniques http://www.aces.edu/dept/fisheries/education/documents/oyster_hatchery_techniques.pdf